Trajectory optimization of a planetary sunshade around L1

A planetary sunshade is a large, reflecting disk built to shield the Earth from a small fraction of solar irradiance and partly compensate global warming caused by greenhouse gas emissions. As a specific form of solar geoengineering, the sunshade is an emergency solution that would be implemented to prevent catastrophic climate change, while working towards the net-zero emission goal. In this paper, a dynamic sunshade is proposed. The motion of the sunshade is designed as a combination of static permanence at two equilibrium points above and below the ecliptic plane to shade the poles and a time-optimal transfer trajectory to connect these equilibrium points without overshading the tropical regions. Such a system is capable of not only reducing the global mean surface temperature anomaly, but also minimizing regional climate changes by tailoring the sunshade’s motion according to climate requirements, which is the primary goal of this work. A simplified climate model is used to evaluate the results of a given shading pattern, directly related to the sunshade’s trajectory. A dynamic sunshade with a radius of 1434 km and orbiting in the vicinity of the Sun-Earth L1 point is able to reduce the global mean surface temperature from 16.39˝C (scenario with 680 ppm of atmospheric CO2, double the amount with respect to the pre-industrial era) to 14.13˝C until equilibrium is reached. It also reduces the polar mean surface temperature (for latitudinal bands above 65˝) by more than 2˝C with respect to a scenario without sunshade and by 0.06˝C with respect to a static sunshade at the displaced L1 point. The optimal results are achieved when the sunshade is located at a distance equal to 30% of the Earth’s radius above and below the ecliptic plane. In addition, the transfers between the equilibrium points start respectively at day 56 and day 250, both measured from the 1st of January.